Silicon carbide devices increase the energy conversion efficiency of power modules; however, the encapsulation process thereof faces unique technical problems. Firstly, the junction temperature Tj of the wafer of current silicon-based power modules is around 150° C., which is still within the acceptable temperature range of common lead-free solder material (Sn3.0Ag0.5Cu). However, when silicon carbide devices are introduced, the junction temperature increases to above 175° C. even for power control module or solar micro inverter of medium to low wattage. The melting temperature of Sn3.0Ag0.5Cn is only 217° C. Therefore, under such temperature conditions, serious creep effect will take place, which is disadvantageous for the mechanical strength of the junction point, causing the junction point unable to satisfy the need for long-term reliability, not to mention products such as electrical modules used in automobiles in applications where the junction temperature reaches 250° C. Therefore, as far as silicon carbide power modules are concerned, high temperature lead-free solders will be one of the key factors in deciding product quality and lifespan.
Currently, the main high temperature lead-free soldering technique includes using zinc-based high temperature lead-free solders, nano-metal powder sintering, and solid-liquid inter-diffusion (SLID).
Different from general reflow or thermocompressive bonding, the working principle of solid-liquid inter-diffusion is to allow low melting temperature solder material to completely react with solid-state high melting temperature metals to form a stable high melting temperature intermetallic compound. This kind of intermetallic compound has a melting temperature of over 300° C.; therefore, it allows the joint to maintain good mechanical strength under high temperature environments, which improves long-term reliability of the module.
However, in application of solid-liquid inter-diffusion, a plurality of technical difficulties involving fabrication and material diffusion arises. For fabrication, problems include junction temperature exceeding 260° C. and fabricating time lasting longer than 10 minutes. These problems can be overcome by solder material and thickness selection, surface treatment, and assembly parameter control. Problems concerning material diffusion can be categorized into fabrication-based and natural behavior, with the prior caused by the difference in assembly temperature of the substrate and wafer, causing uneven diffusion rates across the area of the wafer and substrate, and the latter due to a decrease in volume during the process when the solder material changes into intermetallic compound. Due to the fact that material diffusion is an irreversible chemical reaction, the resulting voids negatively impact the long-term reliability of the power module. Moreover, it should be noted that neither large-area joint nor micro joint prevents the void formation.